The invention relates to optical fibres made of compound glass, to methods of making such optical fibres, and to devices comprising compound glass optical fibres.
Compound glasses have several important properties that cannot be obtained with conventional silicate glasses, e.g. silica, germanosilicate and borosilicate glass, which makes them attractive as alternative materials for optical fibres. Compound, glasses with properties of interest include:                Chalcogenides (e.g. S, Se or Te—based glasses);        Sulphides (e.g. Ge:S, Ga:La:S, As:S, Ge:Ga:S, Ge:Ga:La:S);        Oxy Sulphides (e.g. Ga:La:O:S);        Halides (e.g. ZBLAN (trade mark), ALF);        Chalcohalides (e.g. Sb:S:Br); and        Heavy Metal Oxides (e.g. PbO, ZnO, TeO2).        
Further details of compound glasses can be found in the literature [1]. One compound glass of specific interest is the sulphide glass Ga:La:S, i.e. gallium lanthanum sulphide (GLS), which can transmit in the infrared, has a high refractive index, has a high optical non-linearity and can be doped with over 10% rare earth ions by weight [2]. However, despite the potential of these compound glass materials for optical fibres, their exploitation has been held back by the fact that they are difficult to fabricate to the necessary quality.
Low-loss single-mode compound glass optical fibres are difficult to fabricate, because of the inherent physical and mechanical properties of compound glasses. In particular, to provide the necessary refractive index difference between the core and the clad glasses, changes in the glass composition need to be made. However, different compositions also have different physical behaviour, such as thermal expansion coefficient, glass transition temperature Tg and crystallisation temperature Tx which have to be taken account of in the fabrication process. In particular, the thermal mismatch between the core and clad glasses must be kept to a minimum and needs to be accommodated by the preform design. It is not straightforward to produce small core preforms in a single step.
Another problem that makes it more difficult to make optical fibres from compound glasses rather than silica glasses, is that the temperature window available for preform fabrication and fibre drawing is much smaller. Fabrication of single mode fibre generally requires several heating steps to prepare a precursor preform of suitable geometry. However, the multiple heating steps cycle the temperature, which promotes nucleation site formation at the surfaces of the glass. Nucleation sites lead to crystallisation and result in the final optical fibre being lossy. This is a principal reason for the relatively high transmission losses of current compound glass fibres, e.g. fluoride and sulphide glass optical fibres. It is known that low-loss optical fibres can only be achieved if there is a pristine core-clad interface. Methods such as rotational casting and extrusion can be employed to provide good core-clad interface quality, but are difficult to perform successfully.
In summary, although compound glass fibres are recognised as being desirable in principle, it has not hitherto been possible to fabricate compound glass fibres of sufficient quality to allow widespread device and transmission applications to be realised.